The present application is based on Japanese Patent Application No. 2001-387759 filed Dec. 20, 2001, the content of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a method and apparatus for controlling an operating speed of a motor provided to move an object body, such that a moving speed of the object body coincides with a predetermined target value.
2. Discussion of Related Art
In a conventional printer (e.g., an ink-jet printer) of a type wherein a printing head performs a printing operation on a sheet of paper during a movement of the printing head, an electric motor (hereinafter referred as xe2x80x9cCR motorxe2x80x9d) is used to drive a carriage which carries the printing head. To assure printing actions exactly as predetermined printing positions, a moving speed of the carriage is required to be held constant within a predetermined range or distance of printing. To this end, the moving speed of the carriage is detected by using an encoder, and an electric current to be applied to the CR motor is controlled according to a suitable control algorithm such as a PID algorithm, such that the detected moving speed coincides with a predetermined target value, so that a torque generated by the motor, namely, a drive force required to move the carriage is controlled.
Generally, a direct current (DC) motor is used as the CR motor. It is known that the DC motor has a variation in torque, or so-called xe2x80x9ccoggingxe2x80x9d of torque, which takes place due to a variation in a magnetic attracting force produced between a stator and a rotor of the motor.
This cogging of torque disturbs a predetermined linear relationship between the drive current and the torque of the CR motor. Accordingly, a PID or other closed-loop control of the drive current of the CR motor having the cogging torque has a risk that the torque variation (speed variation) at the cogging frequency is undesirably amplified, resulting in considerable deterioration in the stability of the operating speed of the CR motor.
Described in detail, even when the drive current is adequately controlled so as to control the motor speed to the target value, the detected moving speed of the carriage has an error with respect to the target value at angular positions of the motor at which the torque cogging takes place. Since the drive current is conventionally controlled so as to eliminate this error, the operating speed of the motor cannot be controlled with a high degree of stability.
It is therefore an object of the present invention to control an electric motor provided to move an object body, such that the operating speed of the motor is controlled with high stability, to permit a printing operation with high image resolution, for example. This object may be achieved according to any one of the following modes of the present invention in the form of a motor control method or a motor control apparatus, each of which is numbered like the appended claims and depends from the other mode or modes, where appropriate, for easier understanding of technical features disclosed in the present application and possible combinations of those features. However, it is to be understood that the invention is not limited to those technical features or combinations thereof, and that any one of a plurality of technical features described below with respect to any one mode of the invention may be a subject matter of the present invention, without the other technical feature or features being combined with that one technical feature.
(1) A method of controlling an operating speed of a motor provided to move an object body such that a moving speed of the object body coincides with a predetermined target value, comprising the steps of:
obtaining a speed control error between the target value and a speed value represented by a filtered speed signal obtained by removing a component having a frequency not lower than a predetermined threshold value, from an original speed signal corresponding to the moving speed of the object body; and
controlling the operating speed of the motor according to a control signal generated on the basis of results of a proportional calculating operation and an integral calculating operation of the speed control error, and a result of a differential calculating operation of the original speed signal.
In the motor control method according to the above mode (1) of this invention, the component (hereinafter referred to as xe2x80x9cspecific-frequency componentxe2x80x9d) which has a frequency not lower than the predetermined threshold value and which is included in the original speed signal is removed to obtain the filtered speed signal which is used to obtain the speed control error, which is used for the proportional and integral calculating operations. Accordingly, a periodic variation in the moving speed of the object body is not amplified due to the specific-frequency component, by a feedback control of the operating speed of the motor according to the control signal. On the other hand, the differential calculating operation is performed with respect to the original speed signal which includes the specific-frequency component.
Accordingly, the present motor control method not only prevents deterioration of stability of the moving speed of the object body due to the specific-frequency component included in the original speed signal, but also effectively minimizes minute variations in the moving speed of the object body due to the component having a frequency higher than the predetermined threshold value.
(2) A method according to the above mode (1), wherein the predetermined threshold value is not higher than a cogging frequency of the motor.
The predetermined threshold value of the frequency is preferably selected to be equal to or lower than the cogging frequency of the motor, where the motor is a direct-current (DC) motor, a stepping motor or any other motor which inevitably suffers from cogging of its torque.
(3) A method according to the above mode (1) or (2), wherein the object body is a carriage which carries a printing head.
Where the object body is a carriage carrying a printing head, the present motor control method permits a significant improvement in the quality of an image printed by the printing head.
In a printer, vibrations of various elements of the printer as well as the cogging torque of the motor influence the speed signal. In particular, minute variations in the moving speed (i.e., printing positions) of the carriage generated due to a signal component having a high frequency have a considerable adverse influence on the resolution of the printed image. These minute variations can be minimized by the differential calculating operation which is performed with respect to the original speed signal including such a high frequency component.
(4) A method according to any one of the above modes (1)-(3), wherein the control signal is generated by subtracting the result of the differential calculating operation from a sum of the results of the proportional and integral calculating operations.
(5) A method according to any one of the above modes (1)-(4), wherein the operating speed of the motor is controlled according to the control signal until the object body has reached a predetermined deceleration-start position at which deceleration of the object body is initiated.
(6) A method according to any one of the above modes (1)-(4), wherein the operating speed of the motor is controlled according to the control signal while the object body is moved between a predetermined acceleration-end position and a predetermined deceleration-start position at which acceleration and deceleration of the object body are terminated and initiated, respectively.
(7) A method according to any one of the above modes (1)-(6), wherein the predetermined threshold value is changed depending upon the target value of the operating speed of the object body.
(8) A motor control apparatus for controlling an operating speed of a motor provided to move an object body, the motor control apparatus including a speed-signal generating portion operable to generate a speed signal corresponding to a moving speed of the object body, and a control-signal generating portion operable to generate a control signal for controlling the operating speed of the motor such that the moving speed of the object body represented by the speed signal coincides with a predetermined target value, the control-signal generating portion comprising
a filter operable to remove from the speed signal a component which has a frequency not lower than a predetermined threshold value;
an error calculator operable to obtain a speed control error between a speed represented by an output of the filter and the target value;
a proportional operator operable to obtain a proportional control value proportional to the speed control error;
an integral operator operable to obtain an integral control value proportional to an integral of the speed control error;
a differential operator operable to obtain a differential control value proportional to a derivative of the speed signal; and
an arithmetic operator operable to generate the control signal on the basis of the proportional, integral and differential control values.
In the motor control apparatus according to the above mode (8) of this invention, the speed-signal generating portion is arranged to generate a speed signal corresponding to the moving speed of the object body, and the control-signal generating portion is arranged to generate a control signal for controlling the operating speed of the motor such that the moving speed of the object body represented by the speed signal coincides with a predetermined target value.
Described more specifically, the control-signal generating portion is arranged such that its filter removes from the speed signal a component which has a frequency not lower than a predetermined threshold value, and the error calculator obtains the speed control error between the speed represented by the output of the filter and the target value. The proportional operator obtains the proportional control value proportional to the speed control error, and the integral operator obtains the integral control value proportional to an integral of the speed control error, while the differential operator obtains the differential control value proportional to a derivative of the speed signal. On the basis of those proportional, integral and differential control values, the arithmetic operator generates the control signal for controlling the operating speed of the motor.
Thus, the present motor control apparatus is constructed to practice the motor control method according to the above mode (1), and has substantially the same advantages as the motor control method.
(9) A motor control apparatus according to the above mode (8), wherein the predetermined threshold value is not higher than a cogging frequency of the motor.
The predetermined threshold value of the frequency is preferably selected to be equal to the cogging frequency of the motor, where the motor is a direct-current (DC) motor, a stepping motor or any other motor which inevitably suffers from cogging of its torque, as described above with respect to the above mode (2).
(10) A motor control apparatus according to the above mode (8) or (9), wherein the object body is a carriage which carries a printing head.
Where the object body is a carriage carrying a printing head, the present motor control apparatus permits a significant improvement in the quality of an image printed by the printing head, as described above with respect to the motor control method according to the above mode (3).
(11) A motor control apparatus according to any one of the above modes (8)-(10), wherein the predetermined threshold value of the filter is variable depending upon the target value of the moving speed of the object body.
Where the predetermined threshold value of the frequency varies with the operating speed of the motor, like the cogging frequency of the motor varying with the motor speed, the component having the predetermined threshold must be reduced or suppressed while the object body is moving at the target speed. To this end, it is desirable to change the threshold frequency (cutoff frequency) depending upon the target speed.
(12) A motor control apparatus according to any one of the above modes (8)-(11), further including:
a position detector operable to detect a position of the object body; and
signal switching means for applying the output of the filter to the error calculator while the position of the object body detected by the position detector is within a predetermined constant-speed region in which the moving speed of the object body is held constant at the target value, and for applying to the error calculator the speed signal generated by the speed-signal generating portion, while the position of the object body is within one of accelerating and decelerating regions in which the object body is accelerated and decelerated, respectively.
In the motor control apparatus according to the above mode (12), the speed control error used for the proportional and integral control values is obtained on the basis of the speed signal generated by the speed-signal generating portion, while the object body is moving within the accelerating or decelerating region, and on the basis of the output of the filter which does not include the specific-frequency component, while the object body is moving within the constant-speed region. In this arrangement, a change in the moving speed of the object body is more positively fed back in the accelerating and decelerating regions than in the constant-speed region.
Accordingly, the motor control apparatus according to the above mode (12) is effective to prevent an excessively high rate of change of the moving speed in the accelerating and decelerating regions, so that it is possible to prevent damping in the accelerating period, or an overshoot of the moving speed of the object body upon its movement from the accelerating region into the constant-speed region, assuring a further improvement in the stability of movement of the object body.
(13) A motor control apparatus according to any one of the above modes (8)-(11), wherein the arithmetic operator receives the output of the filter until the object has reached a predetermined deceleration-start position at which deceleration of the object body is initiated.
(14) A motor control apparatus according to any one of the above modes (8)-(13), wherein the arithmetic operator generates the control signal by subtracting the differential control value from a sum of the proportional and integral control values.
(15) A motor control apparatus according to any one of the above modes (8)-(14), further comprising a register which stores data indicative of the predetermined threshold value and which is connected to the filter.
(16) A method of controlling an operating speed of a motor provided to move a carriage carrying a printing head, such that a moving speed of the carriage coincides with a predetermined target value, comprising the steps of:
obtaining a speed control error between the target value and a speed value represented by a filtered speed signal obtained by removing at least a component corresponding to a cogging frequency of the motor, from an original speed signal corresponding to the moving speed of the carriage; and
controlling the operating speed of the motor according to a control signal generated on the basis of results of a proportional calculating operation, an integral calculating operation and a differential calculating operation of the speed control error.
In the motor control method according to the above mode (16), the speed control error for which the proportional, integral and differential calculating operations are to be performed is obtained on the basis of the filtered speed signal obtained by removing at least the component (specific-frequency component) corresponding to the cogging frequency of the motor, from the original speed signal corresponding to the moving speed of the carriage. Namely, the filtered speed signal includes a component having a frequency higher than the cogging frequency.
In the present motor control method, a periodic variation in the moving speed of the carriage is not amplified due to the specific-frequency component corresponding to the cogging frequency, by a feedback control of the operating speed of the motor according to the control signal. Thus, the present motor control method prevents deterioration of stability of the moving speed of the carriage due to the cogging torque of the motor. In addition, the present method is effective to minimize minute variations in the moving speed of the carriage which would take place due to a signal component having a frequency higher than the cogging frequency. Accordingly, the present method permits a significant improvement in the quality of the image printed by the printing head, as described above with respect to the method according to the above mode (3).
(17) A method according to the above mode (16), wherein the component corresponding to the cogging frequency of the motor is a frequency component of the original speed signal, which frequency component is within a predetermined frequency band including the cogging frequency.
(18) A method according to the above mode (16) or (17), wherein the control signal is generated by summing the results of the proportional, integral and differential calculating operations.
(19) A method according to any one of the above modes (16)-(18), wherein the operating speed of the motor is controlled according to the control signal until the carriage has reached a predetermined deceleration-start position at which deceleration of the carriage is initiated.
(20) A method according to any one of the above modes (16)-(19), wherein the predetermined threshold value is changed depending upon the target value of the operating speed of the carriage.
(21) A motor control apparatus for controlling an operating speed of a motor provided to move a carriage carrying a printing head, the motor control apparatus including a speed-signal generating portion operable to generate a speed signal corresponding to a moving speed of the carriage, and a control-signal generating portion operable to generate a control signal for controlling the operating speed of the motor such that the moving speed of the carriage represented by the speed signal coincides with a predetermined target value, the control-signal generating portion comprising:
a filter operable to remove from the speed signal at least a component corresponding to a cogging frequency of the motor;
an error calculator operable to obtain a speed control error between a speed represented by an output of the filter and the target value;
a proportional operator operable to obtain a proportional control value proportional to the speed control error;
an integral operator operable to obtain an integral control value proportional to an integral of the speed control error;
a differential operator operable to obtain a differential control value proportional to a derivative of the speed control error; and
an arithmetic operator operable to generate the control signal on the basis of the proportional, integral and differential control values.
In the motor control apparatus according to the above mode (21) of the present invention, the speed-signal generating portion generates a speed signal corresponding to the moving speed of the carriage, and the control-signal generating portion generates a control signal for controlling the operating speed of the motor such that the moving speed of the carriage represented by the speed signal coincides with a predetermined target value.
Described more specifically, the control-signal generating portion is arranged such that the filter removes from the speed signal at least a component corresponding to the cogging frequency of the motor, and the error calculator obtains the speed control error between a speed represented by an output of the filter and the target value. Further, the proportional operator obtains the proportional control value proportional to the speed control error, and the integral operator obtains the integral control value proportional to an integral of the speed control error, while the differential operator obtains the differential control value proportional to a derivative of the speed control error. On the basis of these proportional, integral and differential control values, the arithmetic operator generates the control signal for controlling the operating speed of the motor.
Thus, the present motor control apparatus is constructed to practice the motor control method according to the above mode (16), and has substantially the same advantages as this motor control method.
(22) A motor control apparatus according to the above mode (21), wherein the arithmetic operator receives the output of the filter until the object has reached a predetermined deceleration-start position at which deceleration of the object body is initiated.
(23) A motor control apparatus according to the above mode (21) or (22), wherein the arithmetic operator generates the control signal by summing the proportional, integral and differential control values.
(24) A motor control apparatus according to any one of the above modes (21)-(23), further comprising a register which stores data indicative of the predetermined threshold value and which is connected to the filter.